Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees C to give the complex stability constants (K-s) and the thermodynamic parameters (Delta G degrees, Delta H degrees, and Delta S degrees) for the stoichiometric 1:1 complexation of light lanthanoid(III) nitrates (La-Gd) with N-benzylaza-16-crown-5 (1) and 15,15-dimethyl-16-crown-5 (3). Using the present and previous data, the effects of substitution and ring enlargement of crown-5 derivatives (1-4) upon complexation behavior are discussed comparatively from the thermodynamic point of view. Possessing a less-symmetrical skeleton and the same N-substitution as compared with N-benzylaza-15-crown-5 (2), N-benzylaza-16-crown-5 (1) gave lower complex stabilities by 1-2 orders of magnitude for all light lanthanoids examined, but exhibited significantly altered relative cation selectivity with much enhanced preference up to 8 for Pr3+ over the other light lanthanoids. The lower complex stabilities for 3 than for the parent 16-crown-5 4 are attributed to the reduced enthalpic gains caused by the steric hindrance between the axial methyl group introduced at C-15 and the accommodated cation, but the methyl substitution appears to help lock the ring conformation such that complexation can occur, as indicated by the positive entropic changes for most light lanthanoids. These results indicate that the introduction of a nondonating sidearm to C- and N-pivot 16-crown-5 alters Significantly not only the binding constant but also the relative cation selectivity.